Staring death in the face: chimpanzees' attention towards conspecific skulls and the implications of a face module guiding their behaviour

Chimpanzees exhibit a variety of behaviours surrounding their dead, although much less is known about how they respond towards conspecific skeletons. We tested chimpanzees' visual attention to images of conspecific and non-conspecific stimuli (cat/chimp/dog/rat), shown simultaneously in four corners of a screen in distinct orientations (frontal/diagonal/lateral) of either one of three types (faces/skulls/skull-shaped stones). Additionally, we compared their visual attention towards chimpanzee-only stimuli (faces/skulls/skull-shaped stones). Lastly, we tested their attention towards specific regions of chimpanzee skulls. We theorized that chimpanzee skulls retaining face-like features would be perceived similarly to chimpanzee faces and thus be subjected to similar biases. Overall, supporting our hypotheses, the chimpanzees preferred conspecific-related stimuli. The results showed that chimpanzees attended: (i) significantly longer towards conspecific skulls than other species skulls (particularly in forward-facing and to a lesser extent diagonal orientations); (ii) significantly longer towards conspecific faces than other species faces at forward-facing and diagonal orientations; (iii) longer towards chimpanzee faces compared with chimpanzee skulls and skull-shaped stones, and (iv) attended significantly longer to the teeth, similar to findings for elephants. We suggest that chimpanzee skulls retain relevant, face-like features that arguably activate a domain-specific face module in chimpanzees' brains, guiding their attention.

elements, especially skulls. I think the authors could substantially improve the work, though, and I will offer some criticisms and suggestion that I think can help them to do this.
First, I should say that I have great sympathy for colleagues who are not native English speakers, but who are required to write in English for most academic journals in their fields. I could not write publishable quality work in Portuguese or Japanese (or in any language other than English). However, the manuscript could be substantially shorter and many points would be clearer if the authors have someone who is a native English-speaker help them with copy editing. Also, eliminating some of the extra words and the repetition would make the work more concise and easier for readers to understand. For example, the last sentence of the Abstract contains five phrases set off by commas that precede a final statement. Much of the wording is redundant and the authors could omit it. Having a single, short sentence that makes a clear statement would be much better. This same logic applies to many other places in the manuscript. Also, the authors should use past tense in reporting their results and then in describing them in the Discussion. This is partly they are reporting on events that occurred in the past. Importantly, also is because when we make statements like "chimpanzees do X", instead of explicitly saying that "in this study, the chimpanzees did X", we seem to be saying that the results apply to all chimpanzees, everywhere and at all times. I have made some copy-editing suggestions below.
More substantively, I would like to see the authors give more attention to the literature on face recognition. The paper is not just about responses to skulls. The authors state in the Discussion (P. 7, line 59) that one of their goals was to assess whether attention to skulls and head-like shapes was "guided by underlying interest in conspecific faces". Some of the work on "interest in faces" has involved investigating how well primates (including humans) can discriminate among individual faces, which was not a question the authors were trying to address. However, I think they would benefit by looking at Taubert's work (Perception, 38: 343, 2009) on "holistic" processing by humans of non-conspecific faces (including those of chimpanzees. She found evidence that humans process chimpanzee faces holistically -as we do faces of conspecifics -but we seem not to do so for spider monkeys or gorillas. I think this has some relevance to the question of whether chimpanzees process skulls, face-shaped (or really head-shaped) objects, or heterospecific faces like they do chimpanzee faces. Likewise, Taubert et al. (2017, Animal Cognition 20:321) found evidence that chimpanzees, like humans, are better attuned to averaged facial images than to specific ones; I suspect that this also has some implications for the authors' work. In a series of experiments that also used eye-tracking, Kano et al. (2018, PLoS One) found that chimpanzees presented with video images looked longer at the mouths than the eyes of conspecifics and of other primate species (so did humans), the opposite of the result from the present study. I realize that the stimuli were different, which might explain the different results. However, I think the authors need to use this paper in contextualizing their study and their results. They could also benefit by noting that Kano et al. found considerable inter-individual variation (although less for chimpanzees than for the other species in their study), some of it related to variation in rearing histories, and by echoing Kano et al.'s emphasis on the fact that many primates attend selectively to conspecific eyes and faces, follow gaze, etc. In fact, we have a big literature on the importance of faces in primate social worlds. (Incidentally, Kano et al, found that bonobos looked longer at eyes than faces; they suggested that this difference from chimpanzees might have occurred because bonobos are more tolerant of eye contact.) I suggest also that the authors look at the methods Kano et al. used, which differed from their own; actually, the general research question was different, because Kano et al. were interested in differences among chimpanzees, bonobos, rhesus, orangutans, and humans, but I think that paper offers valuable information for comparison. Kano and Tomanaga (2010; Animal Behavior 79:227) also deserves attention.
My sense is that the lack of attention to studies like these was because the authors chose to limit their comparisons mostly to the studies of elephants that they cite. Those comparisons are interesting, but I don't think they tell us much that is new about chimpanzees. In turn, this is partly because the authors list "evolutionary thanatology" as one of the manuscript's keywords, but they have not really focused on what their results add to the literature on evolution thanatology other than to note that their results showed that their subjects gave more attention to chimpanzee skulls than to those of other species and that their particular attention to the teeth area is consistent with the idea that this skull region most resembles the faces of living chimpanzees. I would like the authors to say more about what this means for evolutionary thanatology, especially with reference to Goncalves and Carvalho (2019), which I think is one the best contributions to this literature. Doing that in the Discussion instead of repeating results would make the paper more valuable.
I also think the paper would benefit from a table that summarizes the hypotheses and predictions and then notes which predictions were supported. This would make it easier for readers to follow the presentations of all the statistical results. Finally, I would like to see some discussion of the average overall times that the chimpanzees looked at faces, skulls, or stones. I realize that they did not design the study to investigate whether the chimpanzees looked longer at chimpanzees faces than at chimpanzee skulls or stones shaped like chimpanzee faces/heads. We might expect them to look longest at faces, although the face-like aspects of skulls combined with their novelty might have undermined such an effect. Perhaps they could use their data to investigate this issue, although trying to do a statistical analysis may not be valid. In any case, I noticed the following: mean looking time was shorter for the stone than the face or skull in all conditions; it was longer for the face than the skull in the diagonal condition, but the absolute difference was quite small, and it was approximately equal for the face and skull in the frontal condition. I think this deserves some comment. Also, the subjects looked longer at chimpanzee faces than rat faces in only one of three conditions. Was this perhaps because of their experience seeing live rats?
One general comment on methods and reporting of results: I have not yet been convinced that we should abandon p-values. However, if we use conventional statistics and report these values, we should not report "trends". Trends are non-significant, by definition, and we should report the results as "not significant".

6
The first paragraph would benefit from shortening and some re-organization. This also applies to several of the later sections. The entire Discussion could be considerably shorter and more concise. For one thing, you don't need to repeat so many of the results (a table would help; see my comment above). P. 6, line 60, "visible face cues": how could they orient to non-visible cues? I think you mean "cues that are visible on faces". P. 7, lines 1-3: This needs some re-writing. Again, don't invoke "trends". Also, the stones were not "conspecifics"; you mean stones that resembled the heads of conspecifics (or heterospecifics). P. 7, line 4: re-write as "from skulls, which retain multiple face like features, but not objects that resemble only face outlines". P7, line 6: "is" should be "was", and I suggest you re-write this as "The effects were weaker for lateral orientations than for frontal and diagonal orientations for all stimuli, presumably because lateral orientations carry less information about faces". (Skulls & stones do not carry information about facial expressions!) P 7, line 12, "higher looking times": Better as "Looking times were longer for the cat skull…" P. 7, line 25, sentence starting "Finally": "even more decreased" should be "even shorter" and "among" should be "for". P. 7, next sentence: "this higher looking tendency among" should be "the longer looking time for". P. 7, paragraph starting 34: I can understand your point, but please see if you can make it more clearly.
Section 5.2: The material on a proposed "face module" could use some up-dating. Section 5.3: Not all chimpanzees live in rainforests (line 19). P. 8, lines 25-27: Skeletal remains are only collected at research sites, and not at all of them. Also, researchers do not find the carcasses of most deceased chimpanzees, and I suspect that chimpanzees are much more likely to encounter them than researchers are. Also, chimpanzees in the wild have experience handling both the faces (heads) and skulls of species on which they prey; monkeys comprise most prey, and their faces and skulls certainly resemble those of chimpanzees. P. 8, line 34: Teeth are also visible when chimpanzees yawn (very common!), screams, or fight, and they are often visible while individuals are feeding.
Section 6: You don't need to repeat the results. Just give a summary of what you think your results mean.

Review form: Reviewer 3
Is the manuscript scientifically sound in its present form? Yes

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Recommendation?
Major revision is needed (please make suggestions in comments)

Comments to the Author(s)
The manuscript entitled "Staring death in the face: Chimpanzees´ attention towards conspecific skulls and the implications of a face-module guiding this behaviour" presents results from an investigation of how captive chimpanzees attend to images of conspecific skulls, compared to those of non-conspecifics. Having established the state of the art of how elephants respond to conspecific skulls, as well as pointing out a lack of comparable studies on chimpanzees, despite these two species sharing several socio-cognitive traits, the authors justify the rationale of their study as "we aimed to find out, if chimpanzees exhibited the similar interest patterns as observed with African elephants." As their findings, the authors report that chimpanzees pay most attention to images of conspecific faces, followed by those of their skulls and skull-shaped stones. Moreover, chimps scan the conspecific faces and skulls from teeth-region upwards till the nose.
I have two major concerns and a minor one, regarding the theoretical premise of this study, as well as interpretation of its results and their implication. a) In my opinion, it would be better if the authors invoke a theoretical rationale with respect to primate ethology and evolution to justify the choice of the subject of their study. Right now, the rationale sits heavily on elephant-studies. While it is a pertinent rationale on grounds of comparative cognitive research, I believe the manuscript could benefit from bringing in a theoretical framework explaining the adaptive value of an ability to recognise conspecific skulls in their natural environment, for chimpanzees in particular, and primates in general. b) Do the findings of this particular study add anything novel to our existing knowledge on faceperception by chimpanzees? I do not make this remark after having seen the results, on the contrary, the study design itself made me wonder what different results could be expected than from eye-tracking studies on conspecific face recognition by chimpanzees. Are the skull-stimuli, used in this study was a proxy of the face-stimuli. Are the hypotheses a roundabout way to ask "Do chimpanzees look longer at conspecific-faces than non-conspecific ones?" the latter had already been addressed previously, therefore, the authors need to establish the novelty of this study, especially in comparison to extant facial recognition studies on chimpanzees. I think the seed of potentially addressing this concern is already sown by the authors in this manuscript where they reflect, "Not only was this general interest in skulls greater for the conspecific skulls, but the same trend was shown slightly higher for conspecific faces but lesser for conspecific stones, which suggests the main factor for their attraction are faces and particularly conspecific face-like stimuli. What this suggests is that chimpanzees are able to extract familiar face-like features from distinct non-face stimuli such as skulls which still retain said features and fading in conspecific stones where only the outline of a face is shown." This point should be the main pitch of this manuscript, addressing a crucial question, "when does the face-ness of conspecific faces cease to exist for chimpanzees?" In focussing on this particular question, the manuscript potentially could address a very interesting problem in cognitive science, that of perception of essence of objects. The capacity to identify an object, to a limit, even when they have lost some of their component parts, also form building blocks of formation of concepts and has profound implications in larger fields of such as that of cognitive linguistics. c) The third concern is an extension of the above one. It is important that the authors discuss their rationale and findings I the context of previous face-recognition studies, using eye-tracking technology, on chimpanzees. In section 5.2 they discuss about brain areas responsible for facial recognition in primates. However, what remains missing is a more relevant discussion relating to the studies by Fumihiro Kano and colleagues on eye tracking research on chimpanzee face recognition (more so as those studies were largely done in the same facility with common chimps as participants).
Below are my specific comments for respective sections. Since the manuscript did not have line numbers specified, I used the subheadings of the sections, thereafter quoting the particular lines to add my comments on them. Section 2.3 Research motivation & questions: The authors mention, "Moreover, if such interest was guided by some sort of recognition, would chimpanzees exhibit similar looking times towards conspecific faces (hypothesis 2)?" : clarification necessary for the rather vague phrase, 'guided by some sort of recognition', especially while stating a hypothesis to be tested. Section 3.3 Procedure & Stimuli: I understand that the skull-sizes were controlled for, something that the authors rectified after McComb et al 2006. However, my concern lies with dimensions of the non-conspecific skulls. For example, the characteristic formation of the frontal bone and the temporal arches of the non-conspecifics would be markedly different from those of the conspecifics. Therefore, the area of image covered by the stimuli would not be controlled for each species. This particular concern about the area of stimuli presented is somewhat reflected in Fig  2(c) skull condition where the rat-skull takes up more longitudinal area than the others. Fig 3  confirms this, where we see that looking duration on the rodent skull in lateral condition was indeed significantly higher than on the rest. Now, to test hypothesis 1 of this study, the authors are failing their precondition: "With different sets of skulls, all else being the same…", because apart from image size, all other factors are not being the same. On the other hand, does one need to control for the skull-size at all, if the goal is to test whether chimpanzees recognize their conspecific skulls? The difference in skull sizes between non-specifics could perhaps be controlled by including skulls of infant or juvenile chimpanzees into the stimuli-set. My suggestions would be, the authors should justify, their decision to correct for size of the images and not dimensions of the skulls. Section 4.1.4 Teeth & Mouth Regions: The authors find that chimpanzees looked longer at the teeth region of conspecific skulls. This could be an outcome of exposed teeth in fear-grimace facial expressions of chimpanzees, therefore, could be a confounding result. We are not sure if the chimpanzees are paying attention to a particular emotion, i.e. fear indicated by exposed teeth, or the teeth area of a skull.
Section 5.2 Framing the results within face processing research: As pointed out earlier, this sections need reflection on previous eye-tracking studies on face recognition by chimpanzees (e.g. by Kano et al), how finding of the present study relate to previous results, and how this manuscript goes beyond previous findings, thereby establishing its novelty.

Introduction
The authors start their introduction by discussing both chimpanzee and elephant literature. Then suddenly, after the sentence starting "Even more,.." they only refer to elephants and continue referring to non-human animals in general. I would restructure this first part of the introduction, perhaps moving from general findings across taxa to specific chimpanzee-elephant comparisons. At the beginning of page 3, the authors cite Halder & Schenkel 1972 as a "fairly obscure paper". Either elaborate on that point or remove that adjective. Chimpanzees I am a bit confused with this section of the introduction. The author cites two classic experiments but misses the opportunity to cite much more recent and perhaps, relevant work by authors including van Leeuwen, Cronin or Biro to name a few. It seems that most of the literature on chimpanzees' natural encounters with dead bodies is missing. Authors should include crucial references, especially given the limited amount of literature in this field.

Subjects
The authors mention that there were three dropouts. Please state why this was the case. Procedure and stimuli The authors have counterbalanced very well all combinations of condition, sub-conditions, and species. However, I believe the paragraph needs clarification. At first sight, it is hard to find where the 144 trials come from. I would suggest the authors do something similar to what they are already doing in the Statistical Analysis section (e.g., information in brackets: 4 species X 3 conditions X 3 sub-conditions X 4 variations for a total of 144 trials). Out of curiosity, I wonder if researchers considered including human faces to control for familiarity given how unfamiliar they were to all animals except conspecifics. I found a bit unclear which conditions did they received on each testing day. For example, were face and skull conditions always presented together with stones? In other words, days with face and stone and days with skull and stone? I believe this is not the case, and every day they received either skulls or faces or stones, but I am not entirely sure. Statistical Analysis I was puzzled that the authors did not statistically analyze whether chimpanzees look significantly longer to faces vs. skulls vs. skull-shape stones. For instance, if chimpanzees' average looking time is not significantly longer for skulls than skull-shape stones, the significance of the overall results may change.
To do that, the authors could try LMM with a Gaussian error structure. A linear mixed model would allow authors to study in the same model the effects of species, condition (skull, face, skull-shaped stone) and sub-conditions together with any interaction between these variables. The authors could also control for any learning and order effects by including session and/or trial numbers. Such a model would also allow authors to not just control for subject ID as a random effect (as in simple ANOVAs) but also the random slopes between the random effect (subject) and the main effects (e.g. condition, species, sub-conditions). The package the authors use is suitable for the proposed analysis. Notice, though, that I am not stating that the current analyses are incorrect. On the contrary, the authors could extract more information from their data while better controlling the effect of specific variables. Furthermore, the authors would reduce the amount of ANOVAs (eight if I am not mistaken) to just a couple of models focusing on overall looking times and on specific regions of interest within the chimpanzee sub-sample.

Results
Overall looking time I am not entirely sure why the authors need the first paragraph of the result section. If this is a requisite of the journal, it is ok, but it feels like part of the discussion. It is anticipating the results that are later presenter in greater detail with statistical values. This first section also needs some rephrasing in any case. The authors mention that there appears to be a significant trend across skull face and stone conditions regarding stimuli in frontal subconditions. This is right, but then I do not understand why right after this sentence, the authors reiterate that there was more substantial attention to diagonal and frontal (again) sub-conditions in both skull and face but diminished in the lateral sub-conditions. I think it is better to first present the effect on the frontal sub-condition at once. The overall looking time section seems to introduce the results. I would clarify here why the authors only considered diagonal and frontal chimpanzee faces and skulls for investigating the stimuli chimpanzee look at. The authors only explain that by the end of the results section. Skulls and faces In these two results sections, the authors mention, by the end of both paragraphs, that "all other comparisons were non-significant". Do the authors refer to comparisons within the lateral subconditions or in general? The statement seems to be especially unclear for the face sections. Teeth and mouth I wondered what was the reason not to show animal faces (or at least chimpanzee faces for this analysis) with the mouth open so that teeth were visible? Was it due to the potential emotional reactions they elicit (e.g., fear grin face in chimpanzees)? I understand the authors cannot control for eye presence in skulls, but it is possible to show teeth in alive faces. It might be worth clarifying the reasons for future readers.

Discussion
General findings The sentence starting with "Moreover, compared to…" is very unclear. I would recommend reformulating it. Similarly, the sentence discussing the non-significant difference between chimpanzees' and cats' frontal skulls is hard to follow. The authors state that for the lateral skull sub-condition, there were no significant differences among stimuli. Please correct the statement. They seem to look longer for rats than all other species-and there are significant differences between rats and dogs. The end of the second discussion paragraph needs references. Further considerations The end of this section is slightly confusing. It needs rephrasing (e.g., ..they previously seen dead..).

Conclusions
The sentence starting "For H1;.." needs rephrasing. For H2, why is the non-significant trend between chimpanzees and rats' frontal faces likely to disappear with a larger sample size? Curious to know why this is the only trend interpreted that way. Finally, by the end of the manuscript (page 9) the authors discuss that the hypothetical facemodule develop within the context of frontal face-to-face interactions for the first time. Before, they only refer to this module in general terms. Could you elaborate why it evolved in the context of face-to-face interactions? As of now it reads as a conclusion stemming solely form your results.

Review form: Reviewer 5
Is the manuscript scientifically sound in its present form? Yes

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? Yes

Recommendation?
Major revision is needed (please make suggestions in comments)

Comments to the Author(s)
The authors present a novel eye-tracking study in chimpanzees that examines visual attention to pictures of faces, skulls, and stones (that mimicked the shape/color of the species). Four different species of mammals were presented: chimpanzee, cat, dog, and rat, and each condition was presented at three different angles (diagonal, frontal and lateral). As such, this is an impressive dataset for examining heterospecific and conspecific face perception in chimpanzees along three main lines of inquiry: 1) looking times to conspecific versus heterospecific faces (which is not in itself novel, but the inclusion of these particular mammalian species is), 2) whether looking times change with angle of presentation and if so, whether the changes are consistent across con/heterospecifics or not, 3) whether progressive 'degrading' of the face signal (from face to skull to stone) alters looking times, and whether this is consistent across con/heterospecifics, 4) what are the salient features of the presented cues. Unfortunately, rather than using the theoretical framework of face processing here, the authors attempted to ground their research in comparative thanatology, which is not the most appropriate framing here. Consequently, I do not think this manuscript is publishable in its current form. If the authors were willing to substantially revise the manuscript, including the theoretical framework, then I think it could be a publishable contribution. The manuscript should also be edited for clarity/word choices throughout. There are some unnecessarily long and confusing sentences (e.g. the last sentence of 1. Summary). Specific comments: Introduction: As described above, the thanatological framework is weak. The authors themselves note that it is extremely rare for chimpanzees to come across conspecific skulls, so I am a bit confused as to why they chose the justify this study in this manner. While this is worth some treatment in the discussion, there is a far more rich literature on how chimpanzees and other primates process faces that provide appropriate background and justification for the study including: For comparing looking times to con-and heterospecific faces 1) Kano and Tomonaga 2009 -chimpanzee viewing patterns when shown pictures of chimpanzees, humans, other mammals 2) Hattori et al. 2010 -differential sensitivity to conspecific and allospecific cues in chimpanzee and humans Angles of presentation/degradation of signal: 1) Tomonana & Imura 2009  2   . I understand that this is not how they were presented to subjects (to avoid positional biases), but it would be useful to be able to match the stone condition directly to the species/angle of interest.
More information needs to be provided on the statistical methods. What does AOI (speciesstimuli) refer to? Is this, for example, cat-face (vs cat-skull) or cat-frontal versus cat-lateral? Is seems like it is the skull/face/stone category differences later in the paragraph, but then I don't understand how the angle of presentation was analyzed. Please clarify this so that your work is replicable. Results: Please provide descriptive statistics (mean looking times and sds) for all comparisons.
Please also refer to the appropriate figures in the text of the results.
A summary table of results would be much more digestible.

Discussion:
The discussion should be reworked according to the suggested framing above.

Decision letter (RSOS-210349.R0)
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Reviewer comments to Author: Reviewer: 1 Comments to the Author(s)
This is a really important, fascinating and understudied topic, which the authors should be commended for pursuing. The stimuli categories are nicely controlled. I have only minor clarification comments. Why were the particular species of skulls chosen? How were the stones created/selected? Within the main text, the presentation of stimuli needs to be more detailed. How were the stimuli presented -in pairs, one at a time? How many per session? How balanced per order? What did the chimpanzees do on each trial? How many sessions were presented to each subject? Why did three of the subjects drop out?
The structure for the analysis is unclear. Why was AOI one factor -why not species and material (skull, stone)? What are the interactions in the model?
The results are similarly not written up in a clear manner. The authors need to refer explicitly to significant main effects and interactions and specify the factors more clearly. How can there be a significant effect between the DV and AOIS?? (line 53) Why analyze the faces, skulls and stones separately? Was there an interaction that called for splitting the analysis this way? It would have been interesting to see whether there was the same conspecific preference for complete skeletons except for skulls, which would negate the 'face attention' explanation. Why the preference for rat skulls in the lateral condition? I may have missed it but I didn't think the authors addressed this in the discussion. P. 9, line 8, I don't follow the reference to second order relational info Line 32 "where" should be "were." Line 59 missing 'not to?"

Reviewer: 2 Comments to the Author(s)
This manuscript describes an interesting study that contributes to the small literature on how chimpanzees respond to faces and the smaller literature on how they respond to skeletal elements, especially skulls. I think the authors could substantially improve the work, though, and I will offer some criticisms and suggestion that I think can help them to do this.
First, I should say that I have great sympathy for colleagues who are not native English speakers, but who are required to write in English for most academic journals in their fields. I could not write publishable quality work in Portuguese or Japanese (or in any language other than English). However, the manuscript could be substantially shorter and many points would be clearer if the authors have someone who is a native English-speaker help them with copy editing. Also, eliminating some of the extra words and the repetition would make the work more concise and easier for readers to understand. For example, the last sentence of the Abstract contains five phrases set off by commas that precede a final statement. Much of the wording is redundant and the authors could omit it. Having a single, short sentence that makes a clear statement would be much better. This same logic applies to many other places in the manuscript. Also, the authors should use past tense in reporting their results and then in describing them in the Discussion. This is partly they are reporting on events that occurred in the past. Importantly, also is because when we make statements like "chimpanzees do X", instead of explicitly saying that "in this study, the chimpanzees did X", we seem to be saying that the results apply to all chimpanzees, everywhere and at all times. I have made some copy-editing suggestions below.
More substantively, I would like to see the authors give more attention to the literature on face recognition. The paper is not just about responses to skulls. The authors state in the Discussion (P. 7, line 59) that one of their goals was to assess whether attention to skulls and head-like shapes was "guided by underlying interest in conspecific faces". Some of the work on "interest in faces" has involved investigating how well primates (including humans) can discriminate among individual faces, which was not a question the authors were trying to address. However, I think they would benefit by looking at Taubert's work (Perception, 38: 343, 2009) on "holistic" processing by humans of non-conspecific faces (including those of chimpanzees. She found evidence that humans process chimpanzee faces holistically -as we do faces of conspecifics -but we seem not to do so for spider monkeys or gorillas. I think this has some relevance to the question of whether chimpanzees process skulls, face-shaped (or really head-shaped) objects, or heterospecific faces like they do chimpanzee faces. Likewise, Taubert et al. (2017, Animal Cognition 20:321) found evidence that chimpanzees, like humans, are better attuned to averaged facial images than to specific ones; I suspect that this also has some implications for the authors' work. In a series of experiments that also used eye-tracking, Kano et al. (2018, PLoS One) found that chimpanzees presented with video images looked longer at the mouths than the eyes of conspecifics and of other primate species (so did humans), the opposite of the result from the present study. I realize that the stimuli were different, which might explain the different results. However, I think the authors need to use this paper in contextualizing their study and their results. They could also benefit by noting that Kano et al. found considerable inter-individual variation (although less for chimpanzees than for the other species in their study), some of it related to variation in rearing histories, and by echoing Kano et al.'s emphasis on the fact that many primates attend selectively to conspecific eyes and faces, follow gaze, etc. In fact, we have a big literature on the importance of faces in primate social worlds. (Incidentally, Kano et al, found that bonobos looked longer at eyes than faces; they suggested that this difference from chimpanzees might have occurred because bonobos are more tolerant of eye contact.) I suggest also that the authors look at the methods Kano et al. used, which differed from their own; actually, the general research question was different, because Kano et al. were interested in differences among chimpanzees, bonobos, rhesus, orangutans, and humans, but I think that paper offers valuable information for comparison. Kano and Tomanaga (2010; Animal Behavior 79:227) also deserves attention.
My sense is that the lack of attention to studies like these was because the authors chose to limit their comparisons mostly to the studies of elephants that they cite. Those comparisons are interesting, but I don't think they tell us much that is new about chimpanzees. In turn, this is partly because the authors list "evolutionary thanatology" as one of the manuscript's keywords, but they have not really focused on what their results add to the literature on evolution thanatology other than to note that their results showed that their subjects gave more attention to chimpanzee skulls than to those of other species and that their particular attention to the teeth area is consistent with the idea that this skull region most resembles the faces of living chimpanzees. I would like the authors to say more about what this means for evolutionary thanatology, especially with reference to Goncalves and Carvalho (2019), which I think is one the best contributions to this literature. Doing that in the Discussion instead of repeating results would make the paper more valuable.
I also think the paper would benefit from a table that summarizes the hypotheses and predictions and then notes which predictions were supported. This would make it easier for readers to follow the presentations of all the statistical results. Finally, I would like to see some discussion of the average overall times that the chimpanzees looked at faces, skulls, or stones. I realize that they did not design the study to investigate whether the chimpanzees looked longer at chimpanzees faces than at chimpanzee skulls or stones shaped like chimpanzee faces/heads. We might expect them to look longest at faces, although the face-like aspects of skulls combined with their novelty might have undermined such an effect. Perhaps they could use their data to investigate this issue, although trying to do a statistical analysis may not be valid. In any case, I noticed the following: mean looking time was shorter for the stone than the face or skull in all conditions; it was longer for the face than the skull in the diagonal condition, but the absolute difference was quite small, and it was approximately equal for the face and skull in the frontal condition. I think this deserves some comment. Also, the subjects looked longer at chimpanzee faces than rat faces in only one of three conditions. Was this perhaps because of their experience seeing live rats?
One general comment on methods and reporting of results: I have not yet been convinced that we should abandon p-values. However, if we use conventional statistics and report these values, we should not report "trends". Trends are non-significant, by definition, and we should report the results as "not significant".
Specific comments (page numbers refer to pages in upper right of manuscript, e.g. 2 of 12): P. 3, first full sentence: Omit "a fairly obscure paper" and just say " Halder & Schenkel (1972) found that, like elephants, bovids [which species?] responded more to bones of conspecifics than to those of other species." P. 3, line 5: Saying "without any benefit" isn't necessary. How would elephants benefit by interacting with bones of conspecifics? P. 3, last sentence of 2.1: First, say that these authors presented bones to the elephants (and indicate that these were elephants in the wild), not that they presented elephants to the bones. Also, this sentence could be shorter and clearer. "Whereupon" should be "on which". Make a second sentence that is simply "The elephants showed most interest in elephant bones", then a final sentence "Rasmussen (ref.) obtained similar results for captive elephants.
P. 3, 2.2: Omit the first sentence and just say "Two classic studies provided insights into chimpanzee responses to skeletons." P. 5, line 6, "together with stone": Your description of "image groups" and the figure depicting examples of these indicates that a group consisted of either four images of skulls, 4 images that each showed a different species, or four images of skull-shaped stones. "Together with stone" makes it sound like you showed images of stones at the same time as you showed images of faces or skulls. Please make it clear that you showed the stone image groups on every day f testing, but alternated face groups and skull groups. P. 5, line 8: Omit "significant". P. 5, line 15: Missing "were" after "AOIs".
P. 5, 37-39: Omit the material about "trends". Alternatively, the most you can say is that the mean values differed, but not significantly.
P. 5, line 42, sentence, starting "Since": Omit this sentence; you already said this in the Intro.
P. 5, line 43, sentence starting "For analyses": this should be in Methods.
P. 5, line 53: Either say "there was either say there was "a significant relationship" between these variables or that "AOI significantly affected gaze duration". P. 5, line 54: Here and in other places where you present results of statistical tests (especially Tukey results), it would better to write "Overall fixation durations were longer [better word than "higher"] for the chimpanzee skull than for all others (cat: t = -4.18, p = 0.003, Cohen's d=0.83; dog: t = 4.13, p<0.001, Cohen's d=0.84", etc. You already stated that you used Tukey HSD tests; you don't need to repeat that for each comparison. Also, you can "t" instead of "t-ratio" and don't need to repeat "chimp/rat", etc. when you start by indicating that you are presenting the results of comparisons between chimps (o chimp-like stones) and each of the other stimuli.
P. 6, line 5: "positively non-significant" should be "positive, but non-significant". 4.1.2 and later presentation of statistical results: as I urged above, please re-write this to say that you found (or did not find) significant relationships between variables, then state the results of the Tukey HSD tests as I suggested for p. 5, line 54. P. 6, line 19: Omit "once again".
P. 6, line 20: Please do not invoke "marginal" effects. The relationship was non-significant.
P. 6, line 38, sentence starting "Because": This should be in Methods. Also, you don't need to repeat that you ran an ANOVA for the skull regions (line 40); just give the result. Discussion: The first paragraph would benefit from shortening and some re-organization. This also applies to several of the later sections. The entire Discussion could be considerably shorter and more concise. For one thing, you don't need to repeat so many of the results (a table would help; see  my comment above).
P. 6, line 60, "visible face cues": how could they orient to non-visible cues? I think you mean "cues that are visible on faces". P. 7, lines 1-3: This needs some re-writing. Again, don't invoke "trends". Also, the stones were not "conspecifics"; you mean stones that resembled the heads of conspecifics (or heterospecifics). P. 7, line 4: re-write as "from skulls, which retain multiple face like features, but not objects that resemble only face outlines". P7, line 6: "is" should be "was", and I suggest you re-write this as "The effects were weaker for lateral orientations than for frontal and diagonal orientations for all stimuli, presumably because lateral orientations carry less information about faces". (Skulls & stones do not carry information about facial expressions!) P 7, line 12, "higher looking times": Better as "Looking times were longer for the cat skull…" P. 7, line 25, sentence starting "Finally": "even more decreased" should be "even shorter" and "among" should be "for". P. 7, next sentence: "this higher looking tendency among" should be "the longer looking time for". P. 7, paragraph starting 34: I can understand your point, but please see if you can make it more clearly.
Section 5.2: The material on a proposed "face module" could use some up-dating.
Section 5.3: Not all chimpanzees live in rainforests (line 19).
P. 8, lines 25-27: Skeletal remains are only collected at research sites, and not at all of them. Also, researchers do not find the carcasses of most deceased chimpanzees, and I suspect that chimpanzees are much more likely to encounter them than researchers are. Also, chimpanzees in the wild have experience handling both the faces (heads) and skulls of species on which they prey; monkeys comprise most prey, and their faces and skulls certainly resemble those of chimpanzees.
P. 8, line 34: Teeth are also visible when chimpanzees yawn (very common!), screams, or fight, and they are often visible while individuals are feeding.
Section 6: You don't need to repeat the results. Just give a summary of what you think your results mean.

Reviewer: 3 Comments to the Author(s)
The manuscript entitled "Staring death in the face: Chimpanzees´ attention towards conspecific skulls and the implications of a face-module guiding this behaviour" presents results from an investigation of how captive chimpanzees attend to images of conspecific skulls, compared to those of non-conspecifics. Having established the state of the art of how elephants respond to conspecific skulls, as well as pointing out a lack of comparable studies on chimpanzees, despite these two species sharing several socio-cognitive traits, the authors justify the rationale of their study as "we aimed to find out, if chimpanzees exhibited the similar interest patterns as observed with African elephants." As their findings, the authors report that chimpanzees pay most attention to images of conspecific faces, followed by those of their skulls and skull-shaped stones. Moreover, chimps scan the conspecific faces and skulls from teeth-region upwards till the nose.
I have two major concerns and a minor one, regarding the theoretical premise of this study, as well as interpretation of its results and their implication. a) In my opinion, it would be better if the authors invoke a theoretical rationale with respect to primate ethology and evolution to justify the choice of the subject of their study. Right now, the rationale sits heavily on elephant-studies. While it is a pertinent rationale on grounds of comparative cognitive research, I believe the manuscript could benefit from bringing in a theoretical framework explaining the adaptive value of an ability to recognise conspecific skulls in their natural environment, for chimpanzees in particular, and primates in general. b) Do the findings of this particular study add anything novel to our existing knowledge on faceperception by chimpanzees? I do not make this remark after having seen the results, on the contrary, the study design itself made me wonder what different results could be expected than from eye-tracking studies on conspecific face recognition by chimpanzees. Are the skull-stimuli, used in this study was a proxy of the face-stimuli. Are the hypotheses a roundabout way to ask "Do chimpanzees look longer at conspecific-faces than non-conspecific ones?" the latter had already been addressed previously, therefore, the authors need to establish the novelty of this study, especially in comparison to extant facial recognition studies on chimpanzees. I think the seed of potentially addressing this concern is already sown by the authors in this manuscript where they reflect, "Not only was this general interest in skulls greater for the conspecific skulls, but the same trend was shown slightly higher for conspecific faces but lesser for conspecific stones, which suggests the main factor for their attraction are faces and particularly conspecific face-like stimuli. What this suggests is that chimpanzees are able to extract familiar face-like features from distinct non-face stimuli such as skulls which still retain said features and fading in conspecific stones where only the outline of a face is shown." This point should be the main pitch of this manuscript, addressing a crucial question, "when does the face-ness of conspecific faces cease to exist for chimpanzees?" In focussing on this particular question, the manuscript potentially could address a very interesting problem in cognitive science, that of perception of essence of objects. The capacity to identify an object, to a limit, even when they have lost some of their component parts, also form building blocks of formation of concepts and has profound implications in larger fields of such as that of cognitive linguistics. c) The third concern is an extension of the above one. It is important that the authors discuss their rationale and findings I the context of previous face-recognition studies, using eye-tracking technology, on chimpanzees. In section 5.2 they discuss about brain areas responsible for facial recognition in primates. However, what remains missing is a more relevant discussion relating to the studies by Fumihiro Kano and colleagues on eye tracking research on chimpanzee face recognition (more so as those studies were largely done in the same facility with common chimps as participants).
Below are my specific comments for respective sections. Since the manuscript did not have line numbers specified, I used the subheadings of the sections, thereafter quoting the particular lines to add my comments on them. Section 2.3 Research motivation & questions: The authors mention, "Moreover, if such interest was guided by some sort of recognition, would chimpanzees exhibit similar looking times towards conspecific faces (hypothesis 2)?" : clarification necessary for the rather vague phrase, 'guided by some sort of recognition', especially while stating a hypothesis to be tested. Section 3.3 Procedure & Stimuli: I understand that the skull-sizes were controlled for, something that the authors rectified after McComb et al 2006. However, my concern lies with dimensions of the non-conspecific skulls. For example, the characteristic formation of the frontal bone and the temporal arches of the non-conspecifics would be markedly different from those of the conspecifics. Therefore, the area of image covered by the stimuli would not be controlled for each species. This particular concern about the area of stimuli presented is somewhat reflected in Fig  2(c) skull condition where the rat-skull takes up more longitudinal area than the others. Fig 3  confirms this, where we see that looking duration on the rodent skull in lateral condition was indeed significantly higher than on the rest. Now, to test hypothesis 1 of this study, the authors are failing their precondition: "With different sets of skulls, all else being the same…", because apart from image size, all other factors are not being the same. On the other hand, does one need to control for the skull-size at all, if the goal is to test whether chimpanzees recognize their conspecific skulls? The difference in skull sizes between non-specifics could perhaps be controlled by including skulls of infant or juvenile chimpanzees into the stimuli-set. My suggestions would be, the authors should justify, their decision to correct for size of the images and not dimensions of the skulls.
Section 4.1.4 Teeth & Mouth Regions: The authors find that chimpanzees looked longer at the teeth region of conspecific skulls. This could be an outcome of exposed teeth in fear-grimace facial expressions of chimpanzees, therefore, could be a confounding result. We are not sure if the chimpanzees are paying attention to a particular emotion, i.e. fear indicated by exposed teeth, or the teeth area of a skull.
Section 5.2 Framing the results within face processing research: As pointed out earlier, this sections need reflection on previous eye-tracking studies on face recognition by chimpanzees (e.g. by Kano et al), how finding of the present study relate to previous results, and how this manuscript goes beyond previous findings, thereby establishing its novelty.

Reviewer: 4 Comments to the Author(s)
The manuscript presents an interesting finding. It advances the literature on how chimpanzees process face-related information by presenting apes with different species' skulls, faces, and skull-shaped stones. The study also complements previous literature on how chimpanzees react to dead conspecifics. However, although the study presents the results in a very detailed manner, I propose an alternative way to analyze the results (below). Furthermore, the study misses many interesting references related to natural encounters of chimpanzees with dead conspecifics. Finally, the study needs to clarify other points before acceptance. Below, I provide detailed comments. Unfortunately, the article is not number lined; thus, I can only refer to the main sections to target the comments. Summary Break down the very last sentence for a better flow. As of now, it contains six commas, and it is a bit difficult to follow. Introduction The authors start their introduction by discussing both chimpanzee and elephant literature. Then suddenly, after the sentence starting "Even more,.." they only refer to elephants and continue referring to non-human animals in general. I would restructure this first part of the introduction, perhaps moving from general findings across taxa to specific chimpanzee-elephant comparisons. At the beginning of page 3, the authors cite Halder & Schenkel 1972 as a "fairly obscure paper". Either elaborate on that point or remove that adjective. Chimpanzees I am a bit confused with this section of the introduction. The author cites two classic experiments but misses the opportunity to cite much more recent and perhaps, relevant work by authors including van Leeuwen, Cronin or Biro to name a few. It seems that most of the literature on chimpanzees' natural encounters with dead bodies is missing. Authors should include crucial references, especially given The authors mention that there were three dropouts. Please state why this was the case. Procedure and stimuli The authors have counterbalanced very well all combinations of condition, sub-conditions, and species. However, I believe the paragraph needs clarification. At first sight, it is hard to find where the 144 trials come from. I would suggest the authors do something similar to what they are already doing in the Statistical Analysis section (e.g., information in brackets: 4 species X 3 conditions X 3 sub-conditions X 4 variations for a total of 144 trials). Out of curiosity, I wonder if researchers considered including human faces to control for familiarity given how unfamiliar they were to all animals except conspecifics. I found a bit unclear which conditions did they received on each testing day. For example, were face and skull conditions always presented together with stones? In other words, days with face and stone and days with skull and stone? I believe this is not the case, and every day they received either skulls or faces or stones, but I am not entirely sure. Statistical Analysis I was puzzled that the authors did not statistically analyze whether chimpanzees look significantly longer to faces vs. skulls vs. skull-shape stones. For instance, if chimpanzees' average looking time is not significantly longer for skulls than skull-shape stones, the significance of the overall results may change.
To do that, the authors could try LMM with a Gaussian error structure. A linear mixed model would allow authors to study in the same model the effects of species, condition (skull, face, skull-shaped stone) and sub-conditions together with any interaction between these variables. The authors could also control for any learning and order effects by including session and/or trial numbers. Such a model would also allow authors to not just control for subject ID as a random effect (as in simple ANOVAs) but also the random slopes between the random effect (subject) and the main effects (e.g. condition, species, sub-conditions). The package the authors use is suitable for the proposed analysis. Notice, though, that I am not stating that the current analyses are incorrect. On the contrary, the authors could extract more information from their data while better controlling the effect of specific variables. Furthermore, the authors would reduce the amount of ANOVAs (eight if I am not mistaken) to just a couple of models focusing on overall looking times and on specific regions of interest within the chimpanzee sub-sample.

Results
Overall looking time I am not entirely sure why the authors need the first paragraph of the result section. If this is a requisite of the journal, it is ok, but it feels like part of the discussion. It is anticipating the results that are later presenter in greater detail with statistical values. This first section also needs some rephrasing in any case. The authors mention that there appears to be a significant trend across skull face and stone conditions regarding stimuli in frontal subconditions. This is right, but then I do not understand why right after this sentence, the authors reiterate that there was more substantial attention to diagonal and frontal (again) sub-conditions in both skull and face but diminished in the lateral sub-conditions. I think it is better to first present the effect on the frontal sub-condition at once.
The overall looking time section seems to introduce the results. I would clarify here why the authors only considered diagonal and frontal chimpanzee faces and skulls for investigating the stimuli chimpanzee look at. The authors only explain that by the end of the results section. Skulls and faces In these two results sections, the authors mention, by the end of both paragraphs, that "all other comparisons were non-significant". Do the authors refer to comparisons within the lateral subconditions or in general? The statement seems to be especially unclear for the face sections. Teeth and mouth I wondered what was the reason not to show animal faces (or at least chimpanzee faces for this analysis) with the mouth open so that teeth were visible? Was it due to the potential emotional reactions they elicit (e.g., fear grin face in chimpanzees)? I understand the authors cannot control for eye presence in skulls, but it is possible to show teeth in alive faces. It might be worth clarifying the reasons for future readers.

Discussion
General findings The sentence starting with "Moreover, compared to…" is very unclear. I would recommend reformulating it. Similarly, the sentence discussing the non-significant difference between chimpanzees' and cats' frontal skulls is hard to follow. The authors state that for the lateral skull sub-condition, there were no significant differences among stimuli. Please correct the statement. They seem to look longer for rats than all other species-and there are significant differences between rats and dogs. The end of the second discussion paragraph needs references.

Further considerations
The end of this section is slightly confusing. It needs rephrasing (e.g., ..they previously seen dead..).

Conclusions
The sentence starting "For H1;.." needs rephrasing. For H2, why is the non-significant trend between chimpanzees and rats' frontal faces likely to disappear with a larger sample size? Curious to know why this is the only trend interpreted that way. Finally, by the end of the manuscript (page 9) the authors discuss that the hypothetical facemodule develop within the context of frontal face-to-face interactions for the first time. Before, they only refer to this module in general terms. Could you elaborate why it evolved in the context of face-to-face interactions? As of now it reads as a conclusion stemming solely form your results.

Reviewer: 5 Comments to the Author(s)
The authors present a novel eye-tracking study in chimpanzees that examines visual attention to pictures of faces, skulls, and stones (that mimicked the shape/color of the species). Four different species of mammals were presented: chimpanzee, cat, dog, and rat, and each condition was presented at three different angles (diagonal, frontal and lateral). As such, this is an impressive dataset for examining heterospecific and conspecific face perception in chimpanzees along three main lines of inquiry: 1) looking times to conspecific versus heterospecific faces (which is not in itself novel, but the inclusion of these particular mammalian species is), 2) whether looking times change with angle of presentation and if so, whether the changes are consistent across con/heterospecifics or not, 3) whether progressive 'degrading' of the face signal (from face to skull to stone) alters looking times, and whether this is consistent across con/heterospecifics, 4) what are the salient features of the presented cues. Unfortunately, rather than using the theoretical framework of face processing here, the authors attempted to ground their research in comparative thanatology, which is not the most appropriate framing here. Consequently, I do not think this manuscript is publishable in its current form. If the authors were willing to substantially revise the manuscript, including the theoretical framework, then I think it could be a publishable contribution. The manuscript should also be edited for clarity/word choices throughout. There are some unnecessarily long and confusing sentences (e.g. the last sentence of 1. Summary).
Specific comments: Introduction: As described above, the thanatological framework is weak. The authors themselves note that it is extremely rare for chimpanzees to come across conspecific skulls, so I am a bit confused as to why they chose the justify this study in this manner. While this is worth some treatment in the discussion, there is a far more rich literature on how chimpanzees and other primates process faces that provide appropriate background and justification for the study including:   . I understand that this is not how they were presented to subjects (to avoid positional biases), but it would be useful to be able to match the stone condition directly to the species/angle of interest.
More information needs to be provided on the statistical methods. What does AOI (speciesstimuli) refer to? Is this, for example, cat-face (vs cat-skull) or cat-frontal versus cat-lateral? Is seems like it is the skull/face/stone category differences later in the paragraph, but then I don't understand how the angle of presentation was analyzed. Please clarify this so that your work is replicable.

Results:
Please provide descriptive statistics (mean looking times and sds) for all comparisons.
Please also refer to the appropriate figures in the text of the results.
A summary table of results would be much more digestible.

Discussion:
The discussion should be reworked according to the suggested framing above.

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Author's Response to Decision Letter for (RSOS-210349.R0)
See Appendix A.

Are the interpretations and conclusions justified by the results? Yes
Is the language acceptable? Yes

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Recommendation?
Accept as is

Comments to the Author(s)
Thank you for your careful attention to the reviewers' comments in the last round.

Review form: Reviewer 3
Is the manuscript scientifically sound in its present form? Yes

Are the interpretations and conclusions justified by the results? Yes
Is the language acceptable? Yes

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Recommendation?
Accept as is

Comments to the Author(s)
The authors have satisfactorily addressed my questions and concerns with their previously submitted manuscript. I have no further comments on the present one.

Review form: Reviewer 4
Is the manuscript scientifically sound in its present form? Yes

Are the interpretations and conclusions justified by the results? Yes
Is the language acceptable? Yes

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Recommendation?
Accept with minor revision (please list in comments)

Comments to the Author(s)
The authors have significantly improved an already interesting manuscript during the review process. However, I still have a few comments about the interpretation of the results that would need clarification before my final acceptance.
My main comment concerns the interpretation and analysis of the results. It is important to show that chimpanzees fixated significantly more on chimpanzee faces than on chimpanzee skulls. This would support the authors' interpretation stating that chimpanzees may interpret skulls as degraded faces. As of now, it is not easy to see that this has occurred. The interaction effect between type and species in the frontal and diagonal models supports that chimpanzees fixated more in conspecifics than in other species (with some exceptions) for frontal and diagonal faces and skulls, but not for stone-shaped images. But there seems to be no apparent difference in their fixation patterns for chimpanzee faces vs. skulls with other species. One possibility to check faces &gt; skulls &gt; stones (at least in chimpanzees) is to include chimpanzee species and test the main effect of type. Then, using the lme4 package, the authors could use the drop1 function to assess pair-wise comparisons between type levels. Re-leveling the model variables (e.g., species) and using the drop1 function might be better to compare chimps against other species instead of using the Bonferroni correction. Page 19 of 55 (the document I got from RSOS), L 16: In section 3.3, it is unclear that apes experienced only one type of stimuli (face, skull, or stone) within a single image group. The authors wrote, "each image group had three types.", however, it seems that each image group was composed of four pictures, one per species, and all of them were of the same type (either face, skull, or stone)-as shown in Figure 2. Page 19 L 48: When you refer to three independent tests (one for each angle), do you mean three independent LMM models, correct? Page 20 L 7: The presentation of the results is slightly inconsistent across sections. For instance, when the authors state that the "fixation durations were significantly longer overall for the chimpanzees," I would instead state that the fixation durations were significantly longer for chimpanzees compared to or versus other species. The authors already do something similar to what I propose in L 30. Page 21 L 42: Either use versus or versus. Page 21 L 48: The sentence needs to be rephrased "this suggests the stones, with only outlines of each species, were too degraded a stimuli…" Page 21 L 55: The authors suggest that fixation times were slightly higher in diagonal faces compared to diagonal skulls. Do they refer to fixation times for all stimuli or just for chimpanzees? This is the kind of comparison that would support the faces &gt; skulls &gt; stones, at least for diagonal angles, and it would be interesting to know whether those differences are significant. Table 3 seems to support this for species chimpanzee, although there is a high variation for chimpanzee diagonal faces fixation times. Page 22 L 8: I would suggest the authors include the unpublished results in the Electronic Supplementary Material. This analysis supports the authors' interpretation of the lateral fixation times for rat skulls. Page 23 L 10: The sentence needs some revision "one possible explanation is the result was due…" Page 25 L 19: The sentence in brackets is not completely accurate. The frontal conditions had overall longer looking times than diagonal conditions, but for instance the fixation time for diagonal chimpanzee face is longer than for frontal chimpanzee face.

Review form: Reviewer 5
Is the manuscript scientifically sound in its present form? Yes

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Comments to the Author(s)
This revision of the submitted paper is much improved in terms of clarity, especially with regards to the statistical approach and the results. However, I still think it is too heavily framed in the introduction as a thanatological study (which it is not) versus a face perception study. Aside from the abstract/summary, a reader of the introduction as written would assume that they were about to read about a study in which various skulls were placed in a testing area of the PRI and chimpanzees attention to and interaction with those skulls was measured. It is not until the second paragraph of section 2.3 that the reader becomes aware that this is an eye-tracking study, at which point, many readers would think, "Wait, there is a large literature on conspecific and cross-species face perception, including using eye-tracking, so how is this study situated in that literature?". My recommendation would be to completely reframe the introduction to focus on the face-processing and eye-tracking literature that is now not reviewed until the discussion. The link to thanatology is intriguing, but this study does not tell us anything about what chimpanzees know, feel or do in response to death. Given that the main results are 1) chimpanzees prefer to look at chimpanzee faces over dog, cat and rat and 2) that this preference transfers to a partially degraded face signal (skulls), but not to a more degraded signal (stones), and 3) teeth seem particularly salient, I think this paper is more appropriate for a field specific journal, such as Journal of Comparative Psychology or Animal Cognition.
Section and line specific comments are below: 1. Summary (Abstract) This reads as if it is prefacing two studies -one about thanatology and one about face processing Lines 32-33 -Use "hypothesize" or "predict" rather than "suspect" and then state your predictions clearly, otherwise the sentence that begins "supporting our hypotheses" is unclear.
Line 33 -I would use "preferred" or "attended to" rather than "attracted" Line 34-35 -In the sentences with numbered results add the direction of the effect "chimpanzees attended significantly MORE towards conspecific skulls" Line 57-59 -I would remove the last sentence of this paragraph and just add the Halder reference to line 54 as so (Hutson et al., 2013, but see Halder &amp;Schenkel, 1972). Introduction, Section 2.2 (page 3) Line 31 -Insight into what question? Line 32 -"researched the comparative development of chimpanzees and humans" -you do not need to state an author's field of study Line 44-46 -This sentence leaves out an important category of chimpanzee reactions, namely maternal responses to corpses (Biro et al. 2010, Lonsdorf et al. 2020, Carter et al. 2021 The introduction still does not integrate the face processing literature at all. Section 3.3 Procedure Page 5, Line 20 -I know that face-processing researchers are concerned with controlling for the size of stimuli. However, in this case, you should consider prioritizing ecological validity given that you are trying to gain insight into a process that happens in nature. That is, chimpanzee faces and skulls are naturally bigger than cat, mouse and (depending on the breed) dogs, so it's possible that the chimpanzees were attentive to chimpanzees faces and skulls due to the novelty of those items being presented much smaller than the natural size, or at least the natural proportional size to a mouse face/skull.

Results/figures
In figures 2, 4, 5, 6, I recommend reordering the X-axis so that face, skull, stone, is the order going from left to right, which follows the path of degradation Discussion, Section 5 Page 7 Line 19 -Missing word between "find" and "if" Line 26-27 -If I am understanding your statistical results correctly, then you cannot say that there was any significant preference for conspecific skull-shaped stones. Page 8 Line 19-22 -Yes, this is the main finding, that chimpanzee skulls are likely detected as degraded faces. I recommend rewording this as skulls do not present themselves. Section 5.2 -much of this belongs in the Introduction, especially the first four paragraphs Line 36-50 -This paragraph on neuroimaging findings seems outside the scope of this paper, I recommend excluding it. Section 5.3 -while interesting, does not have much relevance for what was actually tested. The only portion that is relevant for this paper is the paragraph on tusks and teeth, but this could be much reduced and included in the face-processing portion. Conclusion &amp; Future Directions, Page 11 I found these two paragraphs much more relevant and explicitly linked to the experimental paradigm, and therefore much more compelling.
Decision letter (RSOS-210349.R1) We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.

Dear Mr Goncalves
On behalf of the Editors, we are pleased to inform you that your Manuscript RSOS-210349.R1 "Staring death in the face: Chimpanzees´ attention towards conspecific skulls and the implications of a face-module guiding this behavior" has been accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referees' reports. Please find the referees' comments along with any feedback from the Editors below my signature.
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Please note article processing charges apply to papers accepted for publication in Royal Society Open Science (https://royalsocietypublishing.org/rsos/charges). Charges will also apply to papers transferred to the journal from other Royal Society Publishing journals, as well as papers submitted as part of our collaboration with the Royal Society of Chemistry (https://royalsocietypublishing.org/rsos/chemistry). Fee waivers are available but must be requested when you submit your revision (https://royalsocietypublishing.org/rsos/waivers). Thank you for the substantial revisions, which have improved the manuscript. As you'll see, the views of several of the reviewers are in favour of publication; however, two of the reviewers offer a range of comments that you'll need to address before the Editors will be prepared to accept the work. In particular, reviewer 5 in the decision letter has a number of substantial concerns regarding the framing of your work in the wider context of existing literature -their concerns regarding the journal's suitability for the work are not criteria on which your work will be judged at this stage, but they make reasonable comments regarding how you are situating your work, which you must address. As two reviewers comment that the work would benefit from further language polishing, please do seek advice from a service such as those available at https://royalsociety.org/journals/authors/benefits/language-editing/ before resubmitting. If you need a little longer than the usual 7-day turnaround, please let the editorial office know, and they will be able to assist. My main comment concerns the interpretation and analysis of the results. It is important to show that chimpanzees fixated significantly more on chimpanzee faces than on chimpanzee skulls. This would support the authors' interpretation stating that chimpanzees may interpret skulls as degraded faces. As of now, it is not easy to see that this has occurred. The interaction effect between type and species in the frontal and diagonal models supports that chimpanzees fixated more in conspecifics than in other species (with some exceptions) for frontal and diagonal faces and skulls, but not for stone-shaped images. But there seems to be no apparent difference in their fixation patterns for chimpanzee faces vs. skulls with other species. One possibility to check faces > skulls > stones (at least in chimpanzees) is to include chimpanzee species and test the main effect of type. Then, using the lme4 package, the authors could use the drop1 function to assess pair-wise comparisons between type levels. Re-leveling the model variables (e.g., species) and using the drop1 function might be better to compare chimps against other species instead of using the Bonferroni correction. Page 19 of 55 (the document I got from RSOS), L 16: In section 3.3, it is unclear that apes experienced only one type of stimuli (face, skull, or stone) within a single image group. The authors wrote, "each image group had three types.", however, it seems that each image group was composed of four pictures, one per species, and all of them were of the same type (either face, skull, or stone)-as shown in Figure 2. Page 19 L 48: When you refer to three independent tests (one for each angle), do you mean three independent LMM models, correct? Page 20 L 7: The presentation of the results is slightly inconsistent across sections. For instance, when the authors state that the "fixation durations were significantly longer overall for the chimpanzees," I would instead state that the fixation durations were significantly longer for chimpanzees compared to or versus other species. The authors already do something similar to what I propose in L 30. Page 21 L 42: Either use versus or versus. Page 21 L 48: The sentence needs to be rephrased "this suggests the stones, with only outlines of each species, were too degraded a stimuli…" Page 21 L 55: The authors suggest that fixation times were slightly higher in diagonal faces compared to diagonal skulls. Do they refer to fixation times for all stimuli or just for chimpanzees? This is the kind of comparison that would support the faces > skulls > stones, at least for diagonal angles, and it would be interesting to know whether those differences are significant. Table 3 seems to support this for species chimpanzee, although there is a high variation for chimpanzee diagonal faces fixation times. Page 22 L 8: I would suggest the authors include the unpublished results in the Electronic Supplementary Material. This analysis supports the authors' interpretation of the lateral fixation times for rat skulls.
Page 23 L 10: The sentence needs some revision "one possible explanation is the result was due…" Page 25 L 19: The sentence in brackets is not completely accurate. The frontal conditions had overall longer looking times than diagonal conditions, but for instance the fixation time for diagonal chimpanzee face is longer than for frontal chimpanzee face.
Reviewer: 5 Comments to the Author(s) This revision of the submitted paper is much improved in terms of clarity, especially with regards to the statistical approach and the results. However, I still think it is too heavily framed in the introduction as a thanatological study (which it is not) versus a face perception study. Aside from the abstract/summary, a reader of the introduction as written would assume that they were about to read about a study in which various skulls were placed in a testing area of the PRI and chimpanzees attention to and interaction with those skulls was measured. It is not until the second paragraph of section 2.3 that the reader becomes aware that this is an eye-tracking study, at which point, many readers would think, "Wait, there is a large literature on conspecific and cross-species face perception, including using eye-tracking, so how is this study situated in that literature?". My recommendation would be to completely reframe the introduction to focus on the face-processing and eye-tracking literature that is now not reviewed until the discussion. The link to thanatology is intriguing, but this study does not tell us anything about what chimpanzees know, feel or do in response to death. Given that the main results are 1) chimpanzees prefer to look at chimpanzee faces over dog, cat and rat and 2) that this preference transfers to a partially degraded face signal (skulls), but not to a more degraded signal (stones), and 3) teeth seem particularly salient, I think this paper is more appropriate for a field specific journal, such as Journal of Comparative Psychology or Animal Cognition. Section and line specific comments are below: 1. Summary (Abstract) This reads as if it is prefacing two studies -one about thanatology and one about face processing Lines 32-33 -Use "hypothesize" or "predict" rather than "suspect" and then state your predictions clearly, otherwise the sentence that begins "supporting our hypotheses" is unclear.
Line 33 -I would use "preferred" or "attended to" rather than "attracted" Line 34-35 -In the sentences with numbered results add the direction of the effect "chimpanzees attended significantly MORE towards conspecific skulls" Line 57-59 -I would remove the last sentence of this paragraph and just add the Halder reference to line 54 as so (Hutson et al., 2013, but see Halder & Schenkel, 1972 Page 5, Line 20 -I know that face-processing researchers are concerned with controlling for the size of stimuli. However, in this case, you should consider prioritizing ecological validity given that you are trying to gain insight into a process that happens in nature. That is, chimpanzee faces and skulls are naturally bigger than cat, mouse and (depending on the breed) dogs, so it's possible that the chimpanzees were attentive to chimpanzees faces and skulls due to the novelty of those items being presented much smaller than the natural size, or at least the natural proportional size to a mouse face/skull. Results/figures In figures 2, 4, 5, 6, I recommend reordering the X-axis so that face, skull, stone, is the order going from left to right, which follows the path of degradation Discussion, Section 5 Page 7 Line 19 -Missing word between "find" and "if" Line 26-27 -If I am understanding your statistical results correctly, then you cannot say that there was any significant preference for conspecific skull-shaped stones. Page 8 Line 19-22 -Yes, this is the main finding, that chimpanzee skulls are likely detected as degraded faces. I recommend rewording this as skulls do not present themselves. Section 5.2 -much of this belongs in the Introduction, especially the first four paragraphs Line 36-50 -This paragraph on neuroimaging findings seems outside the scope of this paper, I recommend excluding it. Section 5.3 -while interesting, does not have much relevance for what was actually tested. The only portion that is relevant for this paper is the paragraph on tusks and teeth, but this could be much reduced and included in the face-processing portion.
Conclusion & Future Directions, Page 11 I found these two paragraphs much more relevant and explicitly linked to the experimental paradigm, and therefore much more compelling.

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Decision letter (RSOS-210349.R2) We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.

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